This invention relates to peristaltic pumps, and more particularly to a novel method and means for increasing the pumping accuracy of such pumps, and even more particularly to peristaltic pumps of the linear variety.
Peristaltic pumps of the type described are particularly suited for use in accurately metering and infusing fluids into the bodies of hospital patients, and the like. The U.S. Pat. Nos. 4,217,933, 4,346,705, 4,299,218, and 4,210,138 disclose one type of fluid infusion and metering equipment which is commonly used today in hospitals and other such institutions where extreme accuracy in the infusion of fluids is very important. This system uses a rotary or roller-type peristaltic pump, as disclosed for example in U.S. Pat. Nos. 4,155,362 and 4,210,138.
One major disadvantage of a system of the type noted above is that it is extremely complicated and expensive. The reason for this is that the peristaltic pump, which is the essence of the equipment, operates in the usual manner repeatedly to compress and expand a section of resilient tubing through which the metered fluid is pumped. This tubing typically is made from a flexible, plastic material such as polyvinyl chloride or the like. As is well known to those skilled in the art, the section of the tubing which passes through the peristaltic pump is intermittently compressed and released at spaced points along its length. It is this alternate expansion and contraction of the tubing, which effects the pumping of the fluid. This is true whether the pump is of the rotary type described above, or of the linear variety, such as shown for example in FIG. 4 of the U.S. Pat. No. 4,155,362.
Regardless of the particular type of peristaltic pump employed, its Achilles' heel is the need for utilizing the flexible tubing, the characteristics of which are subject to change in response to ambient temperature variations, fatigue during prolonged use, tubing eccentricity, etc. Motor speed, the speed of the peristaltic rotor or drive shaft is also a source of error during metering, but this factor (the speed of the rotor) can be controlled very accurately by available, inexpensive control devices. However, the variables in the fluid feed rate that are introduced by virtue of the presence of the plastic tubing have been far more difficult to control.
For example, because of fatigue, tubing elasticity and hence its inner diameter can vary rather drastically during use, and as a consequence the metering rate and pumping efficiency will vary accordingly. In practice, many conventional metering systems of the type described have been found to exhibit as much as a 10% drop in flow rate in a twenty-four hour period. By using rather sophisticated control systems of the type disclosed in the above-noted U.S. patents, it has been possible to reduce this drop to as much as 6 to 7%. However, for many infusion systems even this rather limited drop in the flow rate is undesirable if not intolerable.
It is an object of this invention, therefore, to provide a novel method of stabilizing the pumping rates of peristaltic pumps without requiring the use of any particular types of tubing with the pumps.
Another object is to provide improved metering apparatus of the type described which is capable of substantially reducing the undesirable drop in the flow rate of the apparatus heretofore encountered during prolonged use.
It is a further object of this invention to provide improved metering apparatus of the type described which utilizes relatively simple and inexpensive means for substantially increasing the pumping accuracy of the apparatus.
Still another object of this invention is to provide for fluid infusion systems or equipment of the type described improved peristaltic pump means which is capable of increased pumping efficiency even when employing standard, flexible tubing, rather than utilizing any special type of tubing.
Still other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.